'When you hear hoofbeats, think of horses not zebras' - the old adage is well-known to GPs but what should you do when faced with a zebra, not a horse? Consultant cardiologist Professor Robert Tulloh and GP Dr Louise Tulloh kick off our new series with their advice on how to catch Kawasaki disease in general practice.

Failure to test the formula was downfall of Carr-Hill

A postmortem on the debacle of the Carr-Hill formula has revealed the flaws that resulted in just 2,900 practices out of 10,000 being told their notional list size would increase.

It emerged last week the GPC was still haggling with ministers on the day the contract was published over which factors the formula should include ­ leaving no time to test it properly.

This failure was compounded by inaccurate census data and serious flaws in practice income data provided by primary care organisations.

GPC negotiators revealed Downing Street intervened hours before the contract was published to rule out plans to include an economies of scale factor in the formula, claiming it would offer a perverse incentive for large practices to break up.

Negotiators also wanted the formula to include the extra workload generated by patients with English as a second language and the cost of split-site surgeries, but the evidence base was not robust enough.

The 11th-hour haggling meant there was no time to test the formula on a representative sample of practices.

GPC joint-deputy chair Dr Hamish Meldrum said: 'The statisticians assured us they'd done the modelling and there wouldn't be any losers. They got it wrong and we're having to pick up the pieces.'

There were also fundamental flaws in the data used to calculate practices' global sums.

GPC negotiator Dr Peter Holden told a Norwich roadshow: 'We found out three weeks ago the 2001 census data was flawed. We've mucked you about and I'm sorry.'

GPC chair Dr John Chisholm said practice income data provided by PCOs was also inaccurate.

He added: 'The data that underpinned the global sum was calculated at very short notice and is a year out of date.'

Negotiators believe their final mistake was in the way the formula was presented to practices (see box).

GPC joint-deputy chair Dr

Simon Fradd said: 'We should have instructed statisticians that the

[Carr-Hill] factor had to be at least one. Emotionally it makes GPs feel better.'

The figures that sunk Carr-Hill

GPC negotiators believe the final straw for GPs was the emotional shock when more than 7,000 practices were told their Carr-Hill rating was less than one.